Due to the rapid growth in the area of digital cordless telephony, cordless telephones are fast becoming more than merely home appliances. Recent developments in cordless telephone technology such as phones that support higher data rate and sophisticated applications such as wireless private branch exchanges have made it necessary to develop a digital cordless standard.
One of the first such standards was the Digital European Cordless Telecommunications (DECT) system. DECT is designed as a flexible interface to provide cost-effective communications services to high user densities in small cells. This standard is intended for applications such as domestic cordless telephony, telepoint and radio local loop. It supports multiple bearer channels for speech and data transmission, hand over, location registration and paging. DECT is based on time division duplex and time division multiple access. Gaussian filtered FSK (GFSK) modulation scheme is employed in DECT. GFSK is a premodulation Gaussian filtered digital FM scheme. In order to comply with the standards set out by DECT, a data recovery algorithm must be implemented in the receiver circuit of a cordless telephone. This data recovery algorithm is used in certain applications including a transmitter and a receiver such as a cordless phone where an error may occur when the frequency of the transmitter does not match the expected frequency of the receiver. This frequency shift will turn into a DC shift in the analog waveform, making the data difficult to receive and read.
Existing data recovery algorithms utilize a traditional decision feedback equalizer (DFE) or data slicer which includes both feedforward and feedback paths. Consequently, these type of DFEs require either an analog to digital converter (ADC) and a digital signal processor (DSP) or some analog delay mechanism in order to achieve the desired output. Additionally, traditional applications will utilize an integrating capacitor to acquire the DC level of the incoming signal during the preamble portion of the input signal.
The reference frequency accuracy specifications for DECT allow for a frequency error of up to 100 KHz between two radios. To allow for this error, as well as any residual error in the discriminator's center frequency, an adaptive data slicer threshold is needed. Most DECT radios use a first order linear feedback loop to set up a slicing threshold. The loop is activated during preamble and the synchronization word (sync word), but then the loop is opened once the sync word is detected by the baseband processing and the slice level held on the off-chip capacitor for the remainder of the data packet. There is a trade-off between the initial acquisition and sensitivity to the data pattern. There are only 32 bits available to set up the DC level, including 16 bits of preamble and 16 bits of sync word, which requires a fairly short time constant. However, the sync word is followed by arbitrary data in a field which carries medium access control information. In the few bits processing delay while the baseband recognizes the sync word, the slice level can become corrupted. Traditional circuits suffer quite badly from this process because the tight post detection filtering decreases the amplitude of the preamble relative to the long ‘1’ or ‘0’ sequences.
FIG. 1 is a block schematic diagram of an existing DFE circuit 10 or data slicer. In this circuit 10, an input signal 22 which has been filtered by a Gaussian filter is input to a comparator 12 along with a feedback signal 14. An output signal 26 is coupled with a switch 16 which is operated by a switch control signal 24. When the switch 16 is closed, the output signal is coupled with a coupling resistor 18 and a slicing capacitor 20 which are coupled together in series and grounded 28.
In addition to the aforementioned problems associated with traditional DFE circuits, a user of a radio or phone including a traditional data slicer circuit will oftentimes run into other problems. For instance, the traditional circuit does not exhibit a satisfactory sensitivity or range. Also, traditional data slicer circuits require additional circuitry to operate properly, adding to the complexity and cost of the circuit.